Salicylic acid (SA) is a signal in systemic acquired resistance and as an inducer of the alternative oxidase protein in tobacco cell suspensions and during thermogenesis in aroid spadices (Raskin 1992ab). The "alternative" electron transfer pathway of plant mitochondria is cyanide-resistant (cf. the standard cyanide-sensitive Cyt pathway). The alternative pathway consists of a single ubiquinol oxidase that transfers electrons from reduced ubiquinone to molecular oxygen, producing water as the product. The alternative oxidase does not pump protons across the inner membrane, and because it bypasses the two sites of proton translocation at complexes III and IV, the free energy is released as heat. This promotes thermogenesis during flowering in aroid spadices (Lennon et al, 1997).

Radiolabeling studies with cucumber and potato indicate that ¹⁴C-Phe, ¹⁴C-trans-cinnamic acid, and ¹⁴C-benzoic acid (BA) are all metabolized to ¹⁴C-SA (Meuwly et al, 1995; Coquoz et al, 1998). ¹⁴C-Phe is rapidly metabolized to cinnamate, benzoic acid and SA in potato (Coquoz et al, 1998). In potato the PAL inhibitor 2-aminoindan-2-phosphonic acid inhibits arachidonic acid induced SA accumulation (Coquoz et al, 1998).

In tobacco a benzoic acid 2-hydroxylase is induced by virus-inoculated tobacco (Leon et al, 1993; 1995). Shulaev et al (1995) have demonstrated phloem transport of SA, and labeling of SA from oxygen-18 in the inoculated leaf, consistent with benzoic acid 2-hydroxylase being a monooxygenase. In cucumber cotyledons infection of the cotyledons with tobacco necrosis virus leads to an increase in SA in the upper leaf which results from both synthesis in the upper leaf and transport from the infected cotyledons (Molders et al, 1996). SA accumulation in phloem fluids of cucumber following infiltration of leaves with Pseudomonas syringae pv. syringae cells, is preceded by a transient increase in phenylalanine ammonia-lyase (PAL) activity in stems and petioles (Smith-Becker et al, 1998). In addition to SA accumulation, a second phenypropanoid compound, 4-hydroxybenzoic acid (4HBA) was accumulated in phloem fluids of inoculated cucumber leaves with the same kinetics as SA. It is proposed that 4HBA is derived from trans-cinnamic acid via the intermediate 4-coumaric acid (Smith-Becker et al, 1998). The conversion of trans-cinnamic acid to 4-coumaric acid (an intermediate in flavonoid biosynthesis) is catalyzed by cinnamic acid 4-hydroxlase.

The mechanism of benzoic acid (BA) production from trans-cinnamic acid is unknown, but it may proceed in a manner similar to the B-oxidation of fatty acids (Ryals et al, 1996; Ribnicky et al, 1998).

Both BA and SA can be conjugated to glucose. In potato the SA conjugate has been identified as 2-O-B-glucopyranosylsalicylic acid (Coquoz et al, 1998). Little is known about the regulation of the catabolism of the conjugates, which are inactive in disease resistance (Ryals et al, 1996). Gaseous methyl salicylate (MeSA) is a major volatile in TMV-inoculated tobacco plants, and is produced in parallel with SA. MeSA may represent an airborne defense signal. MeSA is synthesized from SA and apparently acts by being converted back to SA (Seskar et al, 1998). MeSA is a constituent of the floral scent of Clarkia breweri flowers (Dudareva et al, 1998), and is synthesized by an S-adenosylmethionine-dependent salicyclic acid carboxyl methyltransferase that adds a methyl group to the carboxyl group of salicylic acid, not the -OH group.

SA plays a key role in both disease resistance and systemic acquired resistance signaling (Ryals et al, 1996; Shirasu et al, 1997; Delaney, 1997). A mutation in Arabidopsis leads to constitutive expression of systemic acquired resistance (SAR) (Bowling et al, 1994). The cpr1 mutation is recessive and is associated with an elevated endogenous level of SA. This is suppressed in plants producing a bacterial salicylate hydroxylase [EC 1.14.13.1] (nahG), which inactivates SA by converting SA to catechol. It is proposed that the CPR1 gene product acts upstream of SA as a negative regulator of SAR (Bowling et al, 1994). NahG tobacco plants lack both SA and MeSA, and fail to induce PR-1 gene expression in response to MeSA (Seskar et al, 1998).

Potato contains a high basal level of SA in comparison to tobacco and Arabidopsis. This high basal level does not lead to constitutive resistance to Phytophthora infestans because when the bacterial salicylate hydroxylase gene (nahG) is expressed there is no decrease in disease severity, despite drastic reduction in SA levels (Yu et al, 1997). However, NahG plants reduced the ability of arachidonic acid (a natural elicitor produced by P. infestans) to induce SAR to P. infestans (Yu et al, 1997). It is suggested that potato has an ineffective SA signal perception and/or transduction mechanism, and that induction of SAR in potato may involve activation of certain molecular mechanisms that enhance the sensitivity of the plant to SA. In contrast, biologically or chemically induced SAR in tobacco, Arabidopsis and cucumber is associated with enhanced SA biosynthesis (Yu et al, 1997).

SA accumulates both locally and to a lesser extent systemically following inoculation with avirulent pathogens, and exogenous SA induces defense genes and SAR. SA inhibits catalase [EC 1.11.1.6] in vitro, and may have a general affinity for iron-containing enzymes. Binding of SA to heme-containing enzymes may result in the generation of salicylate radicals that might have signal functions in gene activation and cell death (Lamb and Dixon, 1997).

SA enhances hydrogen peroxide production, lipid peroxidation and oxidative damage to proteins in Arabidopsis. SA-enhanced hydrogen peroxide levels are associated with increased activities of Cu/Zn superoxide dismutase [EC 1.15.1.1]. Prolonged SA treatments inactivated catalase and ascorbate peroxidase [EC 1.11.1.11] and resulted in phytotoxic symptoms, suggesting that inactivation of hydrogen peroxide-degrading enzymes serves as an indicator of hypersensitive cell death (Rao et al, 1997). According to Rao et al (1997), SA requires H₂O₂ to potentiate lipid peroxidation, induce PR genes and establish SAR. H₂O₂ stimulates the activity of benzoic acid 2-hydroxylase, and this may be important in signal amplification (Lamb and Dixon, 1997). SA potentiates an agonist-dependent gain control that amplifies pathogen signals in the activation of defense mechanisms (Shirasu et al, 1997). SA induces thermotolerance in mustard (Sinapsis alba) by a pathway that may involve an early increase in hydrogen peroxide (Dat et al, 1998ab).

Low oxygen pressures inhibit programmed cell death in tobacco plants during the TMV-induced hypersensitive response. This offers support to the suggestion that reactive oxygen species (ROS) are involved in signaling or mediating cell death. Induction of PR proteins occurs at low oxygen pressure, and SA accumulation is inhibited about 3-fold (Mittler et al, 1996).

Tenhaken and Rubel (1997) conclude that salicyclic acid is needed in hypersensitive cell death in soybean, but does not act as a catalase inhibitor. Fodor et al (1997) also show that catalase activities were not modified by SA treatment in tobacco.

Transgenic potato plants expressing the bacterial gene encoding glucose oxidase exhibit a constitutively elevated, sublethal levels of H₂O₂, leading to increased disease resistance. The constitutively elevated levels of H₂O₂ activated an array of host defense mechanisms, including a several fold increase in total SA (mostly conjugated SA; free SA levels were unaffected) (Wu et al, 1997).

A high affinity SA-binding protein has recently been described in tobacco and is a candidate for an SA receptor in the induction of plant defense responses (Du and Klessig, 1997). SA activates a 48-kDa MAP kinase in tobacco (Zhang and Klessig, 1997).

For detailed discussions of the role of cell death and SA during systemic acquired resistance see: Greenberg (1997), Lamb and Dixon (1997), Ryals et al (1996), Hammond-Kosack and Jones (1996), and Dangl et al (1996).

Wounding of leaves by chewing insects induces the synthesis of defensive proteinase inhibitor proteins in both wounded leaves and distal unwounded leaves; several chemical signals regulate this response -- oligosaccharides derived from polygalacturonic acid, ABA, auxin, the octadecapeptide systemin, and intermediates of the octadecanoid pathway, including linolenic acid and jasmonic acid. Acetyl salicylic acid (ASA) and SA that are signals for systemic acquired resistance to pathogens, inhibit the wound-induced, JA-induced, systemin-induced and oligosaccharide-elicited accumulation of proteinase inhibitors, by inhibiting the synthesis of JA (Doares et al, 1995). SA blocks the expression of proteinase inhibitor genes in response to primary elicitors of the wound response in tomato leaves. An octadecanoid pathway mutant (JL5) of tomato is compromised in signaling for defense againts insect attack; this mutant is affected in octadecanoid metabolism between the synthesis of hydroperoxylinolenic acid and 12-oxo-phytodienoic acid, and has impaired JA accumulation (Howe et al, 1996).

The induction of plant defensins (low molecular weight, cysteine-rich basic proteins) by pathogens appears to occur via salicylic acid independent pathway, requiring functional components of the ethylene and jasmonic acid response pathways (Penninckx et al, 1996).

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